Field of the Invention
The invention relates to a method for creating a control channel in an optical transmission signal. Further, the invention relates to a device for creating a control channel in an optical transmission signal. Further, the invention relates to corresponding devices for extracting the information in such a control channel of an optical transmission signal.
Description of Related Art
In optical transmission networks, for example a wavelength division multiplex passive optical network (WDM-PON), optical transmission signals are transmitted between a first and a second end of an optical transmission link in order to establish a bidirectional transmission link. In addition to the wanted signal carrying user information, control information must be transported between the ends of the transmission link. Such control information may especially be used in order to tune the wavelength of a tunable optical transmitter in a WDM-PON so that the wavelength of the optical carrier signal created by the tunable optical transmitter corresponds to the center wavelength of the optical WDM channel assigned to the optical transmitter.
A central tuning device has been proposed in EP 2 773 056 which is capable of extracting a tuning information included in an optical transmission signal, which is created by an optical transmitter device at a first or second end of an optical transmission link and included in a control channel of the optical transmission signal, of evaluating the tuning information extracted and of creating a control channel in the optical transmission signal which is transmitted to the respective first or second end of the optical transmission link. This known device or method uses a specific pilot tone frequency which is assigned to the optical transmitter device creating the respective optical transmission signal, wherein the control information is (at least partly) represented by the tone frequency which is modulated onto the wanted signal carrying the user information using simple amplitude modulation. A rather small modulation index is used in order not to deteriorate the signal-to-noise ratio in an inacceptable manner. In this known embodiment of a central tuning device, in which the central tuning device is provided within the WDM path of the WDM-PON, a WDM pilot tone frequency is used for establishing a control channel in each of the optical channel signals that are transmitted to the respective end of the transmission link. The WDM pilot tone is modulated onto the WDM signal, which is transmitted in the respective downstream or upstream direction. In order to transport information in a selected downstream or upstream channel signal the pilot tone can be used as a carrier signal, by applying a digital modulation to this carrier signal.
Thus, in this known method or device an analog amplitude modulation is used in order to create a tuning information in the optical transmission signal, that is the pilot tone, which is digitally modulated if additional digital data are to be included in the control channel. In order to realize such an analog amplitude modulation of an optical signal, additional components are required in an optical transmitter device such as digital-to-analog converters (DAC) or analog-to-digital converters (ADC) or direct digital synthesis (DDS) components. This leads to additional efforts and costs for realizing the hardware that is necessary to establish such a control channel in an optical transmission signal.